FIR checker: differentiate UNSAFE_CALL from INAPPLICABLE_CANDIDATE

To do so, inside the root cause of inapplicable candidate errors,
we will record expected/actual type of receiver, if any.
That will help identifying inapplicable calls on nullable receiver.

compiler/testData/diagnostics/testsWithStdLib/coroutines/kt36947.fir.kt

@@ -1,6 +1,6 @@
 val foo = iterator {
     yield(0)
     val nullable: String? = null
-    nullable.<!INAPPLICABLE_CANDIDATE!>length<!>
-    nullable.<!INAPPLICABLE_CANDIDATE!>get<!>(2)
+    nullable.<!UNSAFE_CALL!>length<!>
+    nullable.<!UNSAFE_CALL!>get<!>(2)
 }

compiler/testData/diagnostics/testsWithStdLib/coroutines/restrictSuspension/outerYield_1_3.fir.kt

@@ -87,7 +87,7 @@ fun test() {
                 this@with.<!UNRESOLVED_REFERENCE!>yield<!>("")
 
                 yield2("")
-                this@with.<!INAPPLICABLE_CANDIDATE!>yield2<!>("")
+                this@with.<!UNSAFE_CALL!>yield2<!>("")
             }
         }
     }